Normal or conventional bulk density thermoplastic materials are commonly melt or heat plasticized and extruded as a molten stream through extrusion dies without considerable difficulty. The continuous single or twin screw extruders employed have rotary screws designed to plasticate the thermoplastic material being processed and to convey the molten polymer to and through a downstream extrusion die at a desired rate.
Conventional extruder screws have feeding, transition, and metering sections successively along their length. The feeding and transition sections are required to provide an over-supply of input molten polymer to the metering section which controls and limits the throughput rate of the over-all extruder. However, should it be desired to process low bulk density thermoplastic materials such as certain forms of polycarbonate or blends of such low bulk density polycarbonate materials with conventional bulk density thermoplastic materials problems are immediately encountered.
As employed herein, the term "conventional bulk density thermoplastic materials" is intended to denote thermoplastic materials having a bulk density in the normal bulk density range encountered with virgin or neat polymer in pellet or bead form. For example, virgin or regular thermoplastic material in pellet or bead form normally have a bulk density greater than about 0.5 grams per cubic centimeter (g/cm.sup.3) and less than about 1.2 g/cm.sup.3, for conventional bulk density polycarbonate materials this is typically about 0.5 g/cm.sup.3 to about 0.75 g/cm.sup.3. As employed herein, the term "low bulk density polycarbonate materials" is intended to denote polycarbonate materials having a bulk density less than about 0.5 g/cm.sup.3. For example, virgin or neat polycarbonate can exist in a regularly shaped low bulk density granular form such as powder or as an irregularly shaped low bulk density form such as agglomerated granules having an appearance resembling "popcorn" and sometimes referred to as flake. Polycarbonate which has been previously processed then mechanically ground can exist in irregularly shaped low bulk density forms sometimes referred to as chip, scrap, regrind, or recycle.
During extrusion of regularly and especially irregularly shaped low bulk density polycarbonate materials or blends of such low bulk density polycarbonate materials with conventional bulk density thermoplastic materials, solids conveying in the screw feeding section is often inadequate to properly fill the screw and hence results in undersupplying the screw plasticating and metering sections. This results in polymer surging, lower throughput rates, high melt temperatures, and excessive polymer degradation. The term "extrudate quality" generally refers to the uniformity of the temperature, pressure, and composition of the extrudate at the dieface. If the extrudate is of low quality, temperature and pressure fluctuations are large and the flow of thermoplastic material through the die will be irregular and unsteady and the quality of the product degraded. An important determinant of an end product is how well its dimensions conform to specification. For example, in polycarbonate sheet variations in thickness greater than .+-.2 percent can degrade end product physical properties such as practical toughness and impact strength, end product aesthetics by causing such problems as undesirable roughness, variations in polish and/or appearance of wrinkles, and processability of the sheet in subsequent molding operations such as vacuum forming and thermoforming.
There have been attempts in the prior art to provide steady throughput rate from the metering section by modifying the feed section of the screw. For example, U.S. Pat. No. 4,461,734 reported a feed section with relatively deeper screw root depth and/or greater screw lead and/or greater screw length than normally employed in feeding sections of extruders handling virgin or high bulk density linear low density polyethylene (LLDPE). Although the modifications assisted in improving the throughput rate of the LLDPE flow throughout the process, a downstream melt pump, sometimes referred to as a rotary gear pump, was required to fully control the throughput rate of the over-all process. Further, the overall energy efficiency of a pump-extruder system may actually be lower than the extruder itself.
While melt pumps will make more uniform the volumetric flow of melted thermoplastic material through the die, experience has shown that passage through a melt pump will not significantly reduce temperature variations in the polymer. Hence a melt pump can only ensure that the volumetric flow rate of the extruder is uniform and if the extrudate has a large temperature variations in it, these will pass through the pump and appear at the die, where they can cause irregular flow and degrade product quality. Various techniques have been employed to minimize the temperature fluctuations such as providing internally heated extruder screws alone or in conjunction with motionless mixers after the melt pump as taught for styrene-butadiene elastomers in EP 0 144 932 B 1.
In view of the continuing need for high extrudate quality during heat plastication and extrusion of low bulk density polycarbonate materials, it would be desirable if there were provided methods and apparatuses which would reduce polymer surging, lower pressure and temperature fluctuations, and reduce polymer degradation while providing good overall throughput rates.